Work out your real fuel economy from the miles you drove and the fuel you used — shown as MPG (US), MPG (imperial), L/100km and km/L together — convert any one of those units into the others, and compare two vehicles' running costs side by side. Enter a distance and fuel, or two odometer readings. Your own numbers; nothing is looked up by make or model, and nothing is sent anywhere.
Units
Distance — enter it directly, or from two odometer readings
If both odometer boxes are filled, their difference overrides the distance box.
Fuel used (added at the refill)
Convert a fuel-economy figure
The other three units are shown below. Remember L/100km is the inverse — lower is better.
The two vehicles
Distance & price
Right box is just the currency symbol shown in results. No exchange rates.
Measured economy reflects how and where you drove — average a few fill-ups for a figure you can trust, and expect it to differ from any factory rating.
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Got your MPG? Estimate a trip's cost.
Once you know your fuel economy, the fuel cost calculator turns it into the cost and fuel for any trip, the cost per mile and your range. Comparing heating fuels by their energy content rather than a vehicle? See the heating fuel cost comparison calculator.
How to calculate gas mileage (miles per gallon)
Fuel economy — what most people call gas mileage, fuel efficiency or fuel consumption — is just distance divided by the fuel it took. To figure your miles per gallon, divide the miles you drove by the gallons you used: MPG = miles ÷ gallons. Drive 300 miles, put in 10 US gallons, and that is 30 MPG. The formula for miles per gallon never changes; the converter and comparison modes are built on the same single division. To find, compute, determine or work out your real economy you only need two honest numbers — a distance and the fuel that covered it — which is why this is a fuel mileage calculator that asks for your own figures rather than a generic rating.
The fill-up method, the reliable way to check your car's MPG
The dependable way to measure economy is the fill-up method. Fill the tank to the first click and note the odometer (or zero the trip meter). Drive normally. At the next fuel stop, fill to the first click again and read off how many litres or gallons the pump added — that is precisely the fuel you used over the distance you covered. Divide one by the other and you have your economy. A single tank is sensitive to how full it was each time, so average three or four fills for a number you can rely on. This calculator accepts either a distance you type or two odometer readings, and the fuel you added.
Convert fuel economy: MPG, L/100km and km/L
Because economy is reported differently around the world, a fuel economy converter is genuinely useful: the US and UK use miles per gallon (on different gallons), most of Europe uses L/100km, and much of Asia and Latin America uses km per litre. This mpg conversion goes through a common base — kilometres per litre — so the figures all reconcile. The one thing to keep straight is direction: MPG and km/L are distance per fuel (higher is better), while L/100km is fuel per distance (lower is better), so they move opposite ways. The consumption converter here uses exact factors rather than a rounded shortcut, so litres-to-gallons and the mileage-per-litre numbers stay consistent.
Exact unit-conversion factors used internally
Quantity
Exact factor
1 mile
1.609344 kilometres
1 US gallon
3.785411784 litres
1 imperial (UK) gallon
4.54609 litres
Compare two vehicles and the fuel savings
An mpg comparison answers a practical question: how much does a thirstier car really cost you? Enter both economies, the distance you cover in a year (or on a trip) and the fuel price, and the tool gives each vehicle's fuel cost and the mileage difference between them. The annual running cost is distance ÷ economy × price for each car, and the fuel-economy savings is simply the gap. Over 12,000 miles a year, 25 MPG against 40 MPG at $3.50 a gallon is roughly $1,680 versus $1,050 — about $630 a year. The price is one you enter, so the comparison reflects your local fuel, not an assumed figure.
A worked example, every unit reconciled
Suppose you drove 300 miles and added 10 US gallons.
30.0 MPG (US) = 300 ÷ 10.
36.0 MPG (imperial) — the same fuel is only 8.327 imperial gallons, so 300 ÷ 8.327.
7.84 L/100km — 37.854 litres over 482.8 km, × 100 ÷ distance.
12.75 km/L — 482.8 km ÷ 37.854 litres.
All four describe one real economy; they differ only because the units differ. Convert 30 MPG (US) in the converter mode and you get exactly these three companions back.
Terms, in plain English
Term
What it means
MPG
Miles per gallon — distance per fuel; higher is better. Differs by US vs imperial gallon.
L/100km
Litres to travel 100 km — fuel per distance; lower is better. The inverse of MPG.
km/L
Kilometres per litre — distance per fuel, metric style; higher is better.
Fill-up method
Measuring economy from one full tank to the next: distance ÷ fuel added.
MPGe
Miles per gallon equivalent — an electric figure against 33.7 kWh per US gallon.
A note for readers across the US, UK and Canada: gas, petrol and gasoline are the same fuel; gas mileage, fuel economy, fuel efficiency and fuel consumption all describe the same idea; and litre/liter and kilometre/kilometer are spelling variants. Choose the units that match how you measured and every figure lines up.
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Frequently asked questions
How do I calculate gas mileage?
Use the fill-up method: fill the tank, note the odometer, drive normally, then refill and write down how much fuel it took. Miles per gallon is the distance divided by the fuel added: MPG = miles ÷ gallons. If you drove 300 miles and put in 10 US gallons, that is 30 MPG. In metric the same trip is litres divided by kilometres times 100 for L/100km, or kilometres divided by litres for km/L. This calculator does all four at once from one distance-and-fuel pair, so you do not have to convert by hand.
What's the fill-up method, step by step?
Fill the tank to the first click and either reset the trip meter or note the odometer reading. Drive as you normally would until you need fuel again. Refill to the first click at the same kind of pump, and record the litres or gallons the pump added — that is exactly the fuel you used over the distance you covered. Divide the distance by the fuel for MPG or km/L, or fuel by distance for L/100km. One tank is a rough figure; average three or four fills for a number you can trust, since a single fill is sensitive to how full the tank was each time.
Should I use the US gallon or the imperial gallon?
Match it to the pump you filled from. A US gallon is 3.785411784 litres and an imperial (UK) gallon is 4.54609 litres, so the same drive gives a different MPG number depending on which gallon you divide by — about 20% apart. A real economy that reads 30 MPG (US) is roughly 36 MPG (imperial). This is why two people with identical cars can quote different MPG. Pick the fuel unit you actually measured in and the tool reports all four economy figures consistently from it.
Why is L/100km lower-is-better and MPG higher-is-better?
They measure inverse things. MPG and km/L are distance per fuel — more distance on the same fuel is better, so a bigger number is better. L/100km is fuel per distance — how many litres to go 100 km — so less fuel is better and a smaller number wins. That is why 5 L/100km beats 8 L/100km, while 50 MPG beats 30 MPG. Because they are inverses, you cannot average MPG figures the way you can average L/100km; convert to a common unit first, which the tool does.
How do I convert between MPG, L/100km and km/L?
Go through a common base. To convert MPG to L/100km, turn miles per gallon into kilometres per litre using the exact mile and gallon factors, then L/100km = 100 ÷ (km per litre). To go the other way, km per litre = 100 ÷ L/100km, then multiply back out to MPG. The converter mode does this for any single figure and shows the other three; it uses 1 mile = 1.609344 km, 1 US gallon = 3.785411784 L and 1 imperial gallon = 4.54609 L, and does not hardcode a rounded MPG-to-L/100km constant, which avoids the small errors those shortcuts introduce.
How do I compare two cars' fuel costs?
Enter each vehicle's economy, the distance you drive (annual or per trip) and the fuel price, and the comparison mode works out fuel used and cost for each, then the difference. Annual cost is distance ÷ economy × price for each car, and the saving is the gap between them. For 12,000 miles a year, 25 MPG versus 40 MPG at 3.50 a gallon is about 1,680 versus 1,050 — roughly 630 a year saved by the more efficient car. The price is yours to enter; nothing is baked in.
Why don't you look up my car's MPG by make and model?
Because a make-and-model figure is a lab rating that rarely matches what you actually get, and those numbers change by trim, year, tyres, load and driving. Measuring your own with the fill-up method gives the real economy of your car as it is driven, which is the whole point. So this tool asks for your own miles and fuel rather than offering a model lookup. If you only want to estimate a trip's cost from an economy you already know, use the fuel cost calculator instead.
What about MPGe for electric, or km per litre for CNG?
MPGe (miles per gallon equivalent) restates an electric vehicle's energy use against a published energy figure of 33.7 kWh per US gallon of gasoline, so a car using 30 kWh per 100 miles is about 112 MPGe. This calculator is for liquid-fuel economy and does not build an EV mode, but the idea is the same distance-per-energy comparison. Compressed natural gas is sometimes quoted in km per litre of an energy-equivalent or in a gasoline-gallon-equivalent; the safest comparison is always on energy or cost per distance using your own measured figures.